Normally, stainless steel alloys are softer than other steel materials, Therefore, they are frequently submitted to a hardening treatment, which basically may be a bulk treatment or a surface treatment. The bulk treatment is intended to harden the steel material homogenously, such as a plate or a wire, throughout the entire cross-section of the material, while the surface treatment is intended to harden only the surface of the component, leaving the substrate substantially unaffected.
For instance, U.S. Pat. No. 5,632,826 (&WO-A-95/09930), which is hereby included in its entirety into the disclosure of the present application by this reference, discloses a precipitation hardened stainless steel in which the strengthening is based on the precipitation of particles throughout the material. The strengthening particles have a quasi-crystalline structure, said structure being essentially obtained at aging times up to about 1000 hours and tempering treatments up to about 650° C. This strengthening involves an increase in tensile strength of at least 200 MPa.
Other processes for precipitation hardening stainless steel and/or components made of said steel are disclosed in WO-A-93/07303, WO-A-01/36699 and WO-A-01/14601, which hereby are all incorporated into the disclosure of the present application by this reference. For example, according to WO-A-01/36699, the production of the material prior to aging/hardening shall be such that the item be subjected to cold forming to a degree of deformation sufficient for obtaining a martensite content of at least 50% preferably at least 70%.
Instead of a hardening treatment affecting the steel throughout and homogenously, in many applications the stainless steel component is provided with a hardened surface, often referred to as “case hardening.” The concept of case hardening is to transform a relatively thin layer of material at the surface of the part by enrichment of carbon or other ingredients, in order to make the surface harder than the substrate, the substrate being the bulk of the steel that remains unaffected by the surface modification.
Stainless steels are often case hardened by carburization. That is a process by which carbon atoms are diffused in solution into the surface of the component. Known case hardening processes are performed at high temperatures. Carburization processes performed at temperatures of about 540° C. or somewhat higher (for stainless steel alloys). However, such temperature processes can promote the formation of carbides in the hardened surface.
Steel tools, wear parts and parts in general with high demands on strength and/or toughness and wear resistance, are often coated to increase their service life and to improve the operational conditions. Known procedures such as CVD or PVD are useful for coating the different parts. The layers used are hard layers which usually are formed by nitrides, carbides or carbonitrides of titanium or hafnium or zirconium or their alloys. The variety of use of primarily coated tools are mentioned in the following publications: “Proceedings of the 13th Plansee-Seminar,” Plansee, May 1993; and “Proceedings of the 20th International Conference on Metallurgical Coatings,” San Diego, April 1993. Moreover, for forming tools, these hard coatings also achieve a reduction in the static friction.
In many mechanical applications, not only the hardness but also the static friction, as indicated above, of the steel surface is a known problem. Even if lubrication is made, the static friction may cause considerable friction loss, especially in cases where a reciprocal movement is present. Examples of such applications are shock absorbers for vehicles, hydraulic systems in the process industry, and internal items of combustion engines, such as cam followers. At high-frequency motion changes, the static friction may cause a local temperature increase on sealing metal surfaces in shock absorbers, which leads to deteriorated performance and risks of leakage of hydraulic oil.
In order to decrease the static friction, exposed surfaces are usually coated with some form of layer with better properties than the under-lying steel substrate. Besides giving a lower friction, one desired property of said layer is to protect against mechanical wear. Therefore, the applied layer should be as hard as possible. In hydraulic steering control equipment in process industry, a high static friction may cause a motion resistance that deteriorates the precision of the hydraulic component. Combustion engines constitute another application, where one endeavors to minimize that static friction. For instance, one critical component is the cam follower for inlet and outlet valves. The surface on which the follower acts is exposed to a very high local load, that may result in serious wear problems.
One conventional way of lowering the static friction and to increase the hardness, is to prepare a very smooth surface and then to apply hard chromium plating on this surface. The hardness level thereby achieved for low alloy wrought steel amounts to about 100 Hv. In order to support the layer, a surface hardening is often made before the hard chromium plating. The process is relatively complicated and involves several positions of the work-piece due to the dimension alterations it suffers during the hardening.
In U.S. Pat. No. 5,830,531 a method is disclosed for coating tools with a hardening and friction-reducing surface layer composition. First, the tool is coated in a vacuum process, such as a PVD procedure, with a first hard coating lying directly on the tool material, and then with a superimposed exterior friction-reducing layer over the hard coating. The grain size of the hard and friction-reducing layers has a linear average width of less than 1 μm, whereby excellent hardnesses and long tool lives are attained. However, in order to achieve the desired hardness, the steel first has to be submitted to a hardening treatment before the coating. The necessity of two treatments make the production more costly.
In U.S. Pat. No. 5,707,748 a method is disclosed which is very similar to the method disclosed in U.S. Pat. No. 5,830,531. The disclosures of these two U.S. patents are incorporated into their entirety into the present disclosure by this reference.
In WO-A-99/55929 a method is described for increasing the resistance to wear of a tool or machine component. According to this patent document, a layered system is provided which is especially designed for tools or machine components that are operated in conditions of insufficient lubrication or dry-running. A treated work-piece consists of a base body or substrate of steel and a hard material layer system next to the substrate, supplemented by a metal layer and finally a sliding layer system, whereby the latter is preferably made of carbide, especially tungsten carbide or chromium carbide, and a dispersed carbon. Although good hardness values and low static friction are achieved, the “composite” system of several layers is complicated, time-consuming and expensive to produce.
Further, in WO-A-01/79585 a DLC (Diamond-Like Carbon) layer system is disclosed for producing a layer system for protection against wear, and improve friction qualities or the like. Said layer system comprises an adhesive layer which is placed on a substrate, a transition layer which is placed on the adhesive layer and an outer layer which is made of diamond-like carbon. The adhesive layer comprises at least one element from the group consisting of the 4th, 5th and 6th subgroups and silicon. The transition layer consists of diamond-like carbon. The layer system has a hardness of at least 15 GPa, preferably at least 20 GPa, and an adhesive strength of at least 3 HF according to VDI 3824 sheet 4. Again, this prior art requires several layers, thereby becoming time-consuming and complicated.
Plasma nitriding is an alternative case-hardening process, which is carried out in a glow discharge in a nitrogen gas-containing mixture at a pressure of about 100 to about 1000 Pa (about 1 to about 10 mbar), and it is one of the used methods to treat stainless steel surfaces, thereby resulting in a nitrogen diffusion layer having high hardness and excellent wear resistance. Nitriding hardening is induced by the precipitation of nitrides in the surface layer. The plasma nitriding is the most recently developed surface hardening procedure and it has already been described in the state of the art. This process replaces traditional nitriding methods, such as gas nitriding and nitrocarburation (short-term gas nitriding, bath nitriding and tenifer (a salt-bath nitriding process sometimes called the “Tuffride process”) treatment), since identical thermo-chemical conditions can be established in this process. Plasma nitriding achieves higher hardness and wear resistance, while creating lower distortion. Furthermore, plasma nitriding is very cost effective. This is due to the fact that subsequent machining, finishing and residue removal processes are frequently not required. Similarly, supplementary protective measures, such as burnishing, phosphatizing, etc., may not be necessary.
The plasma nitriding is performed in a vacuum furnace. Treatment temperatures in the range of about 400 to about 580° C. are employed, subject to the requirements of the process in question. Typical treatment temperatures are in the range of about 420 to about 500° C. Treatment times vary between about 10 minutes and about 70 hours, depending upon the component to be treated as well as desired structure and thickness of the layer(s) formed. The most commonly used process gases are ammonia, nitrogen, methane and hydrogen. Oxygen and carbon dioxide are used in the corrosion-protective step of post-oxidation. Besides the type of process gas used, pressure, temperature and time are the main parameters of the treatment process. By varying these parameters, the plasma nitriding process can be fine-tuned to achieve the exact, desired properties in any treated component.
Any iron-based material can be submitted to plasma nitriding. The process does not require the use of special types of nitriding steel. Moreover, the results attained by plasma nitriding can be reproduced with pinpoint accuracy. This is especially important in the manufacture of serial products. However, plasma nitriding does not significantly reduce the static friction.